1. Field of the Invention
The present invention relates generally to the field of geothermal aqueous liquid production and more particularly to methods for preventing scale formation by such geothermal aqueous liquids.
2. Discussion of the Prior Art
Large subterranean reservoirs of naturally occurring steam and/or hot aqueous liquid can be found in many regions of the world. These natural reservoirs of geothermal steam and water or brine are particularly prevalent in regions where the thermal gradient near the earth's surface is abnormally high, being most often found in regions of volcanic, geyser and fumarole activity, for example, along the rim of the Pacific Ocean.
When readily accessible in advantageous locations, geothermal steam and water or brine have, for some time, been used for therapeutic purposes, industrial processes and/or direct heating. Although current interest is further developing geothermal resources for such purposes still exists, principal effort has recently been directed more towards developing these resources, which are usually considered to be at least partially renewable, for production of electric power, the use of which is usually far less site-restricted than is the more direct use of the geothermal fluids for non-electric power purposes. In particular, the recent steep increases in hydrocarbon fuel costs and actual or potential shortages of heretofore abundant supplies of hydrocarbon fuels, together with increasing bias against nuclear power, have substantially accelerated interest in developing geothermal fluids for electric power generation.
General techniques are, of course, known whereby hot geothermal fluids can be used to generate electric power. Geothermal steam can be used, usually after treatment to remove particulate matter and such gases as carbon dioxide and hydrogen sulfide, in substantially the same manner as boiler-generated steam to drive combination steam turbine/electric generator apparatus. Pressurized geothermal water or brine, having a temperature above about 400.degree. F., can be flashed to a lower pressure to extract steam used for driving steam turbine/generators. Lower temperature geothermal water or brine can be used in a closed loop, binary system to vaporize a working fluid, the vapor being used to drive a gas turbine/generator.
Use of geothermal steam, being relatively direct and hence comparatively inexpensive, is usually preferred over use of geothermal water/brine for production of electric power, and in places such as The Geysers in California, a substantial amount of electric power is currently being generated at a competitive cost by geothermal steam. However, the scarcity of geothermal steam as compared to geothermal water/brine in commercially usable amounts has resulted in much of the geothermal power developmental efforts being concentrated on the use of geothermal water/brine.
In spite of the known, general techniques for using hot geothermal water or brine for production of electric power, in actual practice the problems encountered with handling and disposing of the large amounts of usually heavily contaminated and frequently highly saline geothermal liquids have often been quite formidable. Development of geothermal water/brine resources for production of commercial amounts of electricity has, in consequence, often been very difficult and costly to achieve.
The most serious problems encountered with use of hot geothermal aqueous liquids, especially with use of geothermal brine, for producing electric power (and often for other uses as well) usually result from severe equipment scaling caused by the liquid. Because of their typically high temperatures and their long natural residence times in subterranean formations, geothermal aqueous liquids ordinarily leach large amounts of minerals from the formations. These leached minerals typically include heavy metals such as lead, zinc, iron, silver, cadmium, and molybdenum. Such other minerals, as calcium and sodium, typically in the form of chlorides, are also typically dissolved into the geothermal liquid, as are naturally occurring gases, including carbon dioxide, hydrogen sulfide and methane. Large concentrations of silica, which may be in the form of silicic acid oligomers, are also commonly found dissolved in hot geothermal aqueous liquids.
During geothermal water/brine extraction, that is, while the fluid is leaving the producing formations and is still in the extraction wells, as well as during subsequent use of the fluid, pressure and temperature changes occur which cause saturation levels of many of the dissolved materials to be exceeded. As the saturation levels of these dissolved materials are exceeded, the materials leave the solution and, depending on the materials involved, may form insoluble, scale-forming precipitates. Other mechanisms, such as chemical reactions under altered water/brine conditions and polymerization, may also lead to formation of scale in system lines and equipment.
In typical geothermal power plants operated by steam obtained by flashing geothermal water/brine to a reduced pressure, several relatively distinct types of scaling problems may be encountered. For example, scaling within the producing formations, extraction wells and lines and equipment up to, and through, the wellhead separators used to separate non-condensable gases from the water/brine, is largely attributable to the formation of metal sulfides by reactions between dissolved metals and hydrogen sulfide. Downstream thereof, during, and subsequent to, flashing of the water/brine to remove steam, the predominant scaling is typically that caused by silica, due to normal polymerization processes and/or reactions with iron to form insoluble, iron-rich siliceous material. Calcite scaling, however, tends to predominate in the power plant extracted steam portion, including in the steam turbine. The scaling mechanisms responsible for the several scaling problems are different from one another, and the rate at which scale builds up on the inside of pipes and equipment may vary, not only according to scale type but according to composition, flow rate and temperature of the water/brine, from only fractions of an inch per month to several inches per month.
Because of the high cost and great difficulty of removing most scale deposits from pipe and equipment, considerable effort has understandably been expended in developing processes to prevent this often severe scaling. In general, these efforts have been directed either towards developing processes for controlled removal of the scale-forming materials, for example, for removal of silica by seeding techniques or by chemical addition before the scale can be formed, or towards developing processes for maintaining the scale-forming materials in solution during the entire geothermal water/brine transit through the power plant or other system. Processes for keeping the scale-forming materials in solution during the water/brine transit typically include pressure and/or temperature control of the water/brine and addition of chemicals which inhibit scale-forming mechanisms and/or reactions.
Particular disadvantages of induced precipitation of scale-forming materials, particularly of silica, are that large, complex, and usually quite expensive, precipitation equipment is necessary and that the cost of disposing of the large quantities of induced precipitates, which may amount to many tons per power plant operating day, is high. As a result, processes for preparing scale formation by maintaining the scale-forming materials in solution between geothermal water/brine extraction and reinjection or other disposal at least appear to be preferable over those processes which induce precipitation.
In developing processes for keeping scale-forming materials in solution, many types of scale formating mechanisms have been found to be quite pH-sensitive. Geothermal water/brine is usually fairly acidic in its natural state, typically having a room temperature or "effective" pH of between about 3.5 and about 4.5 or 5. (Unless otherwise stated, pH as used herein is the effective pH). Acidizing the geothermal water/brine somewhat more has been found to inhibit formation of the most common scales. Providing additional hydrogen ions during the acidizing is believed to enhance reactions which compete with precipitation reactions and/or to shift equilibrium precipitation reactions away from precipitation. Very importantly, each of the above-mentioned metal sulfide, calcite and silica scaling, certain of which predominate in different regions of typical geothermal water/brine power plants, are considered to be controllable by reducing the pH of the water/brine.
At its above mentioned typical pH, corrosion caused by geothermal water, or more specifically, by geothermal brine, to associated delivery and use piping and equipment has not usually been found to be a very serious problem, even when using piping and equipment made from common steels. As a result, the small pH reduction necessary to control scaling by the geothermal brine is not expected to significantly increase corrosion problems in the brine delivery and use system as a whole. However, in actual practice severe corrosion has sometimes been found to occur, at least in relatively localized regions at and near acid injection points. Such corrosion seems to result from concentrated acids being added so as to effect the desired brine pH reduction in as short a brine flow distance as practical when typically high brine flow rates are involved. As a result, considerably more expensive and much less readily available corrosion resistant piping and equipment may be required in such regions. In addition, expensive, highly corrosion resistant pumps and equipment are, of course, necessary for injecting the acid into the brine, which may, for example, be pressurized between about 100 psig to over 400 psig.
Further complications and expenses are involved with acidizing the brine in close proximity to the underground production zone so as to reduce sulfide scaling in the well. At the high production zone temperatures, typically between about 400.degree. F. and 600.degree. F., the corrosion rate is substantially increased and at these temperatures virtually any type of metal acid-delivery system will rapidly corrode. However, the severe flow turbulance associated with high brine production rates has precluded use of other than metal acid-delivery systems.
An object of the present invention is, therefore, to provide a method for acidizing hot geothermal water/brine so as to reduce or substantially prevent scaling thereby.
A further object of the invention is to provide a method for reducing the pH of hot geothermal water/brine, to reduce or substantially prevent scaling thereby, without causing substantial corrosion problems.
Another object of the invention is to provide a method for preventing or substantially reducing scaling by hot geothermal water/brine by acidizing the water/brine while it is still in the extraction well and is in close proximity to the production zone.
Additional objects, advantages and features of the invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawing.